In recent years the technology based on OLED (organic light-emitting diodes) has become so well established in the field of screen technology that the first commercial products based on these are now available. In addition to screen technology, OLEDs are also suitable for use in surface lighting technology. For this reason, intensive research is being conducted toward the development of new materials.
As a rule, OLEDs are realized as layered structures, which consist primarily of organic materials. For better understanding, a simplified structure is shown as an example in FIG. 1. The core of such components is the emitter layer, in which as a rule emitting molecules are embedded in a matrix. In this layer, negative charge carriers (electrons) and positive charge carriers (holes) meet and combine to form so-called excitons (=excited states). The energy contained in the excitons can be released from the corresponding emitters in the form of light, wherein in this case the term “electroluminescence” is applied. An overview of the function of is given in H. Yersin, Top. Curr. Chem. 2004, 241, 1 and H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials,” Wiley-VCH, Weinheim, Germany, 2008.
Since the first reports on OLEDs (Tang et al. Appl. Phys. Lett. 1987, 51, 913), this technology has undergone continuous further development, especially in the area of emitter materials. Whereas the first materials using pure organic molecules were able to convert a maximum of the use of phosphorescent compounds made it possible to circumvent this fundamental problem, so that at least theoretically, all excitons can be converted into light. These materials as a rule are transition metal complexes, in which the metal is obtained from the third period of the transition metals. Here, primarily very expensive noble metals such as iridium, platinum or gold are used. (See also H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422). In addition to the costs, the stability of these materials is also partially disadvantageous for their use.
A new generation of OLEDs is based on the use of delayed fluorescence (TADF: thermally activated delayed fluorescence or singlet harvesting). In such cases, for example, it is possible to use Cu(I) complexes, which because of the small energy distance between the lowest triplet state T1 and the singlet state S1 (ΔE(S1−T1) located above it, triplet excitons can return to a singlet state thermally. In addition to the use of transition metal complexes, pure organic molecules can also utilize this effect.
Some of such TADF materials were already used in the first optoelectronic components. The solutions to date, however, have drawbacks and problems: the TADF materials in the optoelectronic components often do not have adequate long-term stability, thermal stability or chemical stability against water and oxygen. In addition, not all important emission colors are available. Furthermore, some TADF materials are not vaporizable and thus are not suitable for use in commercial optoelectronic components. In addition some TADF materials do not have materials suitable energy situations relative to the other materials used in the optoelectronic component (e.g., HOMO energies of TADF emitters of greater than or equal to −5.9 eV). Sufficiently high efficiencies of the optoelectronic components at high current densities or high light densities cannot be achieved with all TADF materials. In addition, the synthesis of some TADF are expensive.